Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
In more than the one hundred years that electrical energy has been distributed to household installations and industrial installations, there has been a continuing development and improvement of safety. Initially, the main safety hazard posed by an electrical distribution system (EDS) was fire. As the electrical distribution systems developed the distribution voltages also increased, and the danger of electrocution and electrical shock, particularly when AC voltages were used, became another major concern.
The electrical distribution systems in all industrialized countries, and in particular low voltage (LV) networks and household installations, are earthed for safety reasons. The intention of such “earthing” is to allow the use of electrical protection devices as part of the distribution system to protect against electric shock and electrocution. The electrical protection devices provide “electrical protection” which, in LV networks is required to perform two major functions:                The prevention of electrocution or serious electric shock to personnel who come into direct or indirect contact with live electric conductors.        Following the failure of any electrical insulation of equipment being supplied electrical power by the distribution system, the prevention of: damage to electrical equipment; and possible fire ignition.        
Given the above, various earthing systems have been developed over the last century. The three main types of earthing systems recognized internationally are:                TT system: where T is indicative of a direct connection of one point to earth. That is, an “earth-earth” system.        TN system: where T is indicative of a direct connection of one point to earth; and N is indicative of a neutral. That is, an “earth-neutral” system.        IT system: where T is indicative of a direct connection of one point to earth; and I is indicative of all live parts being isolated from earth. That is, an “isolated from earth” system.        
A more detailed description of these power supply systems with earth connections can be found in the IEC 60364-1: 2005 Standard.
The terms “IT system” or “IT network” are used interchangeably within this specification, unless for any given occurrence the context clearly indicates to the contrary, to refer to an earthing methodology for an electrical distribution system or an electrical distribution system using that earthing methodology. Where reference is made in this specification to an information technology system or network, use is made of the abbreviation “an IT&T system” or “an IT&T network”.
In a number of industrialized countries the TN system is used widely. The earth path or exposed conductive parts are supplemented by connection to the distribution neutral. This is called the multiple earthed neutral system or, more usually, the MEN system. This system utilizes the supply utility neutral to improve—that is, to reduce—the earth path resistance. An earth stake included at an electrical installation is connected to the utility neutral at the installation switchboard and the utility neutral is also connected to earth at regular intervals along its path to the substation transformer. In TN systems the integrity of the earthing of the installation depends on the reliable connection of the earth stake.
An insulation fault in a TN system turns into a short circuit fault, and the fault has to be removed by pre-installed protection devices. The typical protection devices used are residual current devices (RCDs).
While the modern approach in TN systems to personnel and equipment safety protection is to use a low resistance earth connection between the source (the supply utilities) and the load (the household installation), the often large earth resistance variation at different locations within the distribution system and the installation provides varying levels of protection to personnel and equipment. In some cases the general mass of earth may have inherently high resistance—such as when the earth stake is in dry sandy soil or sandstone—which will inherently and undesirably increase the earth resistance.
TN (or MEN) networks are often considered a cheap and reliable mechanism for the delivery of power from a source of production to a point of consumption. However these networks are not without risk. For example, even in a relatively low population centre such as Australia each year tens of people die from exposure to electricity. It is also the case that TN systems are prohibited in certain military and mining installations involving explosives or other volatile materials due to the risk of detonation posed by such networks. Also a TN network, due to having extremely high fault currents, is susceptible, when a fault occurs, to giving rise to fire damage or other thermal damage. Environmentally, a TN network produces more electrical noise than other networks, primarily due to the harmonics generated. It is usually the 3rd harmonic that is most problematic, and which proves very expensive to moderate or obviate. The evidence arising out of the Australian experience with TN networks indicates that that electrical faults contribute to:                10% of all building fires.        1000's of hospital admissions from electrical shocks.        10,000's of damaged and destroyed electrical appliances.        
There are two types of faults that are most common in TN systems. Both fault types are “earth faults” in that the fault current path is between the active conductor and earth. These two types are:                A short circuit fault: where the fault path electrical impedance is low and the resulting fault current extremely high. These high fault currents should trigger the over-current protection quickly and limit the potential for fire generation and for significant equipment damage.        A high impedance fault: where the fault path electrical impedance is high and the fault current relatively low. These low currents may be below the tripping level of any installed over-current protection and can persist for long periods of time without detection, ultimately giving rise to risks of arcing and thermal damage.        
The second of these, the high impedance fault, is the most hazardous type in terms of potential equipment damage, even though the fault current is lower.
As the great majority of fault types that cause electric shock and electrocution are “earth faults”, involving a fault path between active conductor and earth. One partial solution is to make use of an IT system. Such as system is also referred, in general terms, as an unearthed system or a system having electrical separation—for example, see Australian and New Zealand standard AS/NZS 3000:2007-7.4.1. An IT system is recognized by International and Australian standards as a possible means of providing protection against electric shock and electrocution caused by “earth faults”.
An IT system prevents hazardous electric shock by removing the earth from the installation, hence being known as an “unearthed” system. Other common names include a floating system or an isolated system. As an “unearthed” system removes the earth connection between the source and the load, there is no closed circuit for fault current, whether electric shock current or fault over-current, to return to the source of supply. If there is no earth connection, then the fault current is very low, for the only “earth” path is through a capacitance path between the general mass of earth and the metal casing. This gives a very high impedance path and hence very low fault current. The fault current levels are typically so low as to not pose a risk of electric shock or heating and fire ignition.
Unearthed systems were used in the first half of the 20th century to gain the benefit of greater reliability of supply. This reliability arises from the ability to more robustly accommodate faults. For example, a single fault in one part of the distribution network need not shut the power down to the remainder of the network, as there is no fault current to operate an over-current protection device. More recently, IT systems are used less in distribution and more so in installations where reliability of supply is critical. Examples of such installations include industrial installations such as aluminium smelting and semiconductor fabrication plants, hospital operating theatres and certain office buildings.
The unearthed or floating system is able to be used in an installation notwithstanding it is fed from a distribution network having a TN or TT system. It does require, however, that the network and the installation are isolated from each other by an earth-free isolation transformer. Low voltage isolating transformers are referred to as Leakage Protection Devices (LPDs) and are the preferred method of protection over RCDs in operating theaters and areas outlined in Australian and New Zealand standard AS/NZS 3003:2003, which relates to medical wiring.
IT systems suffer from a number of disadvantages. For example, if there occurs a first fault which creates a high impedance path to earth, that first fault is able to exist for a long time without being detected for there is no over-current condition. If there subsequently occurs a second fault that creates a short circuit to earth—which is a likely consequence of the first fault remaining unresolved—there will be a high fault current with all the same touch potential problems as occur in TT and TN system. For this reason International Standard IEC 60364-4-41 requires IT systems to have insulation monitors for providing a visual indication of the presence of the first fault.
International and Australian Standards also limit the use of IT systems to only one item of Class 1 equipment per isolation transformer/generator unless there is additional protection by way of automatic disconnection in case of a fault. Such additional protection includes an RCD. However, RCDs also have disadvantages due to, amongst other things, a reliance on regular trip testing to validate their operation. In some surveys of RCDs in the UK and USA it has been found up to 10% of the RCDs were not in operating condition when tested. Moreover, RCDs rely on having an electrical path to carry the residual current that may be generated. If there is no earth path, the RCD will not have any residual current to operate. For those applications where an earth connection is not established, or not readily available—such as portable electrical generation equipment such as generators and inverters on worksites and temporary locations—the use of RCDs may leave personnel and equipment at risk. Moreover, RCDs are unable to detect DC residual current.
Even where an RCD is correctly installed and operating, and all the preconditions for successful operation are available, the available commercial products are typically slow to operate and allow large fault currents to flow before isolation of the fault occurs. Both the size and duration of these currents greatly increase the risk of electrocution and electrical injury to a person or persons causing or in contact with the fault.
Given the above limitations of RCDs, when use is made of an IT system in an installation, modifications have to be made to allow the RCD to provide the required protection. In effect, this results in the installation being converted from an IT system into a TN system. While some of the reliability benefits are able to be gained following the conversion, it defeats the purpose of the improved safety features for protection of personnel and equipment of having an IT system.
Other protection circuits are also used in a TT EDS, such as a voltage detection safety device (ELCB). These devices were designed primarily for a TT EDS and required an earth path for correct operation. Accordingly, when included in a domestic installation, for example, having an earth electrode, the devices were part of a load circuit that was earthed via that earth electrode. In some jurisdictions, the load circuit was also required to have a second earth electrode grounded approximately 2 meters from the first and which was directly connected to the ELCB itself. For example, see Australian Standard AS3000-1981. This wiring configuration was a partial solution to cure the spurious tripping of the ELCB in response to electrical faults in nearby load circuits such as those of adjacent houses. Other spurious tripping events included power surges or nearby lightning strikes. Other limitations of ELCBs include:                Faults that do not pass through the earth wire to the earth electrode are not detected.        A single building is not easily split into multiple load circuits with independent fault protection.        Being susceptible to false triggering by external voltages carried by infrastructure connected to the earthing system. An example of such infrastructure is metal pipes.        
In more recent years, and in an attempt to address the disadvantages mentioned, above, the RCD has generally replaced ELCBs. While an RCD operates on fault current and is regarded as a superior technology it, in turn, and as mentioned above, provides incomplete protection and is also subject to spurious tripping.